In the production of oil and gas, it is a constantly occuring problem that sand is torn loose from the formation, accompanying the oil and/or gas into the well. Sand following the flow of oil or gas, so-called produced sand, may cause great damage on production equipment, both within and outside the well.
The amount of produced sand increases with increasing flow rate, and an important measure against sand production is, thus, to limit the flow rate. Measuring equipment has been developed in order to detect produced sand, so that sand-reducing measures may be carried out prior to the production equipment has been damaged to such a degree that it has to be repaired.
Additionally, it is common to strengthen formations easily giving away sand, e.g. by injecting sand to which a binding agent, e.g. sand/gravel admixed an adhesive agent of the type referred to as "glue gravel". Also, known filters of a plurality of types are installable in the well in order to prevent loose sand from accompanying the flow up through the well.
After some time of use, the lower part of the well is filled with loose sand, and it becomes increasingly difficult to maintain an acceptable flow rate. Upon maintenance of a well, it is, therefore, usual to try to remove loose sand gathered at the bottom of the well. Loose sand may also result from drilling or other work within the well, and one tries to remove such sand before production start.
A common way of removing sand is to circulate liquid within the well. Liquid pumped down towards the bottom through a pipe string, conveys sand to the surface upon return through the annulus between pipe string and well wall. It is known to add a gas, e.g. nitrogen, to the liquid in order to increase the effect, but it has been found to be difficult to remove sand satisfactorily in this manner.
Also, it is known to fetch up sand by means of a container lowered down into the well to be filled with sand before it is hoisted up from the well. The container is closed at surface level (before being lowered), so that it will contain air or other gas at atmospheric pressure when it arrives to the lower area of the well where the removal of sand is going to be carried out. At the bottom of the well, the container is opened. This may e.g. happen in that an actuation means included in the release mechanism for the closure body of the container inlet which may be assigned a return device towards closed position, strikes against the well bottom face, thus opening the inlet for a flow of liquid therethrough, bringing sand with it. The well pressure which is substantially higher than the pressure within the container, causes an intense but brief flow of sandy liquid into the container. When the container is lifted out of the well and the actuation means of the release mechanism no longer is resting itself against the well bottom face, the inlet is closed and the sand conveyed up to the surface, kept within the container. The disadvantage of this known apparatus and the filling method of the container is that the degree of filling becomes small and that removal of sand therefore takes much time. When the container partly filled with sand has reached surface level, where the sand is to be discharged, this takes place through the inlet of this known apparatus, the inlet, thus, acting as outlet in this phase of the process.